The market for portable electronic devices, such as portable audio devices and digital still cameras, have rapidly grown in recent years. With this trend, secondary batteries, which can be reused by charging them, have come to play an important role in supplying power to such portable electronic devices. Among these batteries, secondary batteries which are to be used as drive power sources for electronic devices are required to increase their capacity in order to increase the operating time of the electronic devices.
The increase in the capacity of a secondary battery, however, greatly affects the safety of the secondary battery. Specifically, to charge the secondary battery, the secondary battery is fitted to a charger. At this time, heat generated from the charger or heat generated by the secondary battery itself causes the temperature of the secondary battery to rise above an ambient temperature. The extent of this temperature rise is generally about 10° C. When the capacity of the secondary battery is increased, however, the extent of the temperature rise increases accordingly. Thus, when a strong shock or the like is applied from outside to a fully charged secondary battery having an increased temperature, for example, this secondary battery might abnormally generate heat. Such a phenomenon might also occur when the secondary battery is charged at a high ambient temperature.
To solve the above problem, Patent Document 1 discloses the following method. In this method, to charge a battery safely, the temperature of a secondary battery which is being charged is monitored such that when the temperature of the secondary battery exceeds a given temperature, charge current is switched to a small value, thereby suppressing a temperature rise of the secondary battery which is being charged.
Patent Document 2, for example, discloses a method for controlling charging with charge current and charge time adjusted based on a detected temperature of a secondary battery.